Propargyl-PEG8-NHS ester is a PEG derivative containing a propargyl group and an NHS ester group. The hydrophilic PEG spacer increases solubility in aqueous media. This reagent is an amine reactive reagent for derivatizing peptides, antibodies, amine coated surfaces etc. The propargyl group can be reacted with azide-bearing compounds or biomolecules via copper catalyzed azide-alkyne Click Chemistry to yield a stable triazole linkage.